1. Field of the Invention
This invention relates to an apparatus adapted to enable and/or to improve ambulation in persons afflicted with weakness or instability in the knee joint and in particular, with an apparatus that does so without maintaining the leg locked in extension during the entire gait cycle. The apparatus is designed to be especially useful for patients afflicted with Duchenne muscular dystrophy, but may also be used by patients who have suffered spinal cord or nerve root injuries or suffer from neurological or muscular disease and have a similar pattern of proximal weakness to that suffered by patients with muscular dystrophy, and also by patients with severe ligamentous damage at the knee who experience secondary loss of control of the knee.
2. Description of the Prior Art
Patients having Duchenne muscular dystrophy experience a loss in the power of the quadriceps muscles, and hence, knee control. Such patients, to compensate for weak quadriceps and gluteus maximus muscles, adopt a characteristic posture and gait which is awkward and leads to other problems. Specifically, these patients often develop lumbar lordosis, equinus posturing and anterior pelvic tilt. In the extreme the lordosis brings the line of force posterior to the hip. The equinus moves the distal end of the line of force forward. This in turn changes the usual torques about these joints.
In terms of the knee, the usual torque which tends towards flexion becomes a torque favoring extension. The ligaments then provide the limit on knee extension and the locked knee becomes a firm, albeit awkward support for ambulation. The conventional form of orthosis maintains the leg in extension during gait.
While the straight-legged brace is often prescribed, mechanical orthoses in which the knee is alternately mechanically locked and unlocked have been developed, in some cases as a spin-off from prosthetic technology.
The functioning of prior art mechanical orthoses and the difference and improvement over such represented by the present invention can be better understood with reference to the positions in dorsiflexion and plantar flexion assumed by the ankle during a normal gait cycle. FIG. 1, derived from data generated and reported by D. H. Sutherland in Gait Disorders in Childhood and Adolescence (Waverly Press, Inc. 1984), represents graphically the motion of a normal adult ankle during a gait cycle measured from foot strike to foot strike on the same side. The dotted vertical lines represent, respectively from left to right, foot flat, opposite foot strike and toe off.
As shown in FIG. 1, the ankle, during one gait cycle, goes through relative plantar flexion twice. To approximate normal gait, the leg should swing freely at the knee well before toe off, i.e., when the ankle is in a state of relatively small plantar flexion.
One recent approach to providing an orthosis for persons afflicted with weakness or instability of the knee is exemplified in U.S. Pat. No. 4,456,003 in which a pivotally mounted dog alternately interferes with a rotatable cog member, the axis of rotation of which corresponds to that for elongate members extending along the upper and lower parts of the leg and corresponding to a hypothetical pivot axis for the knee. This orthosis uses a unicentric hinge which does not closely approximate the axis of the knee.
The device disclosed in U.S. Pat. No. 4,456,003 is not a locking-type orthosis, i.e., the knee is not locked into extension during part of the gait cycle. Instead, a tension member is used so that a moment acting to extend the leg is created. The tension member is released by plantar flexion. As the person plantar flexes the ankle before swinging it forwardly through the air, the dog is withdrawn from its interfering relationship with the cog.
Since a single angle in plantar flexion releases the knee, the selection of this angle is problematic in that it must be selected to be greater than the plantar flexion undergone by the ankle shortly after foot strike and before foot flat.
Also the encouragement of plantar flexion may encourage the undesirable development of contractures. The encouragement of dorsiflexion, on the other hand, is considered therapeutic, especially in patients affected with Duchenne muscular dystrophy.
Another orthosis is described in U.S. Pat. No. 2,883,982. This orthosis provides a polycentric hinge including a gear means and a locking mechanism which is automatically released during dorsiflexion. With reference again to FIG. 1, the release of the locking mechanism during dorsiflexion would occur before the 50% point during the gait cycle i.e., before opposite foot strike. This is considered too early as weight and balance have not been established on the opposite foot.
Yet another orthosis is shown in French Pat. No. 948,372 which employs a unicentric hinge and locking mechanism which appears to be released during dorsiflexion. This orthosis would be subject to the same shortcomings as the orthosis described in U.S. Pat. No. 2,883,982.